Electro-optical device

ABSTRACT

A liquid-crystal electro-optical device is offered which can operate at high speeds and easily oriented. The value of the surface tension of liquid crystal-orienting layers is 40 dynes/cm or more, and these layers are rubbed in antiparallel directions to each other. This reduces the pretilt angle of the molecules of a nematic liquid crystal sandwiched between two substrates. The anisotropy of the dielectric constant of the nematic liquid crystal is positive.

This application is a Division application of Ser. No. 09/076,819, filedMay 13, 1998 now U.S. Pat. No. 5,995,185; which itself is a Division ofSer. No. 08/572,074, filed Dec. 14, 1995 now abandoned; which itself isa Division of Ser. No. 08/081,705, filed Jun. 25, 1993, now U.S. Pat.No. 5,495,355.

FIELD OF THE INVENTION

The present invention relates to an electro-optical device using anematic liquid crystal which shows high responsiveness and highcontrast.

BACKGROUND OF THE INVENTION

Conventionally, twisted-nematic liquid-crystal electro-optical deviceshave been used as display devices for watches, electronic calculators,and so on. The structure of such a twisted-nematic liquid-crystalelectro-optical device is now described briefly by referring to FIG. 5.A nematic liquid crystal whose dielectric constant has positiveanisotropy is injected between two substrates 51 and 52 which areoriented at 90° with respect to each other. Thus, liquid-crystalmolecules 53 are twisted. When an electric field is applied to thisliquid crystal, interaction of the field with the anisotropy of thedielectric constant orientates the long axes of the liquid-crystalmolecules at right angles to the substrates. The twisted condition ofthe liquid-crystal molecules when no voltage is applied to the liquidcrystal is discriminated from the condition in which the voltage isapplied, by the use of a pair of polarizer plates 54. Alternatively, anematic liquid crystal whose dielectric constant has negative anisotropyis provided between a pair of substrates which have been subjected to avertical orientation treatment.

In recent years, great progress has been made in the research onferroelectric liquid crystals. An optical device using a ferroelectricliquid crystal is fabricated by orienting the molecules in twosubstrates, bonding together these substrates with a spacing of about 2μm that is considerably narrower than the spacing in a twisted-nematicliquid crystal, and injecting a liquid crystal between the substrates.When no electric field is applied, the ferroelectric liquid-crystalmolecules have two stable states. When an electric field is applied, themolecules are oriented and settle in one state. When an electric fieldof a reverse sense is applied, the molecules are oriented and settle inthe other state. Both dark and bright conditions are produced bydiscriminating these two states of the liquid crystal through the use ofa polarizer plate.

The response time of an optical device using this ferroelectric liquidcrystal is very short, or approximately tens of microseconds, andoptical devices of this kind have been expected to find wideapplication. Also, active-matrix types in which switching elements suchas TFTs or MIMs are arranged at pixels are available. Furthermore,supertwisted-nematic liquid crystals in which a nematic liquid crystalis twisted at 180-270° are obtainable.

However, the response times of the aforementioned twisted-nematicelectro-optical devices are very long, or tens of milliseconds. Also,the steepness of the response to the applied voltage is poor. Therefore,their application is limited except for display devices having smallareas such as watches and electronic calculators. In order to improvethe response time, a decrease in the spacing between the substrates maybe contemplated. If the spacing is narrowed, the time taken to bring theliquid crystal from ON state to OFF state (hereinafter referred to asthe rise time) is shortened but the time taken to bring the liquidcrystal from OFF state to ON state (hereinafter referred to as the falltime) is not shortened. In addition, it is difficult to induce a90°-twist in the liquid-crystal molecules between the two substrates.

Although the response may be enhanced by increasing the driving voltage,an appropriate range of voltages for driving the liquid crystal isdetermined by the used liquid crystal. Therefore, it is not easy toincrease the voltage.

Indeed electro-optical devices using ferroelectric liquid crystals showshort response times, but numerous problems exist. First, it is verydifficult to control the orientation of the liquid crystal. To controlthe orientation, rubbing, oblique deposition of silicon oxide, a methodusing a magnetic field, a temperature gradient method, and other methodshave been heretofore employed. At present, however, it is impossible toobtain a uniform orientation by any of these methods. Consequently, highcontrast cannot be derived.

Secondly, what can be used as a ferroelectric liquid crystal is a liquidcrystal showing smectic phase. Accordingly, the ferroelectric liquidcrystal has a layer structure intrinsic in the smectic liquid crystal.Once this layer structure is destroyed by an external force, theoriginal state cannot be regained even if the external force is removed.To regain the original state, it is necessary to heat the liquidcrystal, for transforming it into an isotropic phase. In this way, theferroelectric liquid crystal is not practical because its layerstructure is destroyed by a very weak external impact.

Thirdly, in a ferroelectric liquid crystal, electric charge isaccumulated at the interface with an orienting film because ofspontaneous polarization of the liquid crystal itself, thus developingan electric field opposite in sense to the polarization of the liquidcrystal. Therefore, if the same image is kept displayed for a long time,this image will linger after it is attempted to display the next image.

Fourthly, the contrast ratio of an electro-optical device using aferroelectric liquid crystal depends much on the tilt angle (cone angle)of the liquid crystal. It is known that the tilt angle (cone angle)providing the greatest contrast ratio is 22.5° (45°). Although liquidcrystals satisfying only the above requirement, i.e., the tilt angle(cone angle) is 22.5° (45°), have been already synthesized,ferroelectric liquid crystals which can also meet other importantconditions, e.g., a temperature range in which the liquid crystal showsferroelectricity and the response to AC pulses, have not been yetdeveloped. Therefore, at present, greater emphasis is placed on theabove-described temperature range than the tilt angle. For thesereasons, the contrast ratios of electro-optical devices usingferroelectric liquid crystals, which are yet presently in anexperimental stage, are not very high. Today it is very difficult to usea ferroelectric liquid crystal as a display device because of theproblems described above.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a liquid-crystalelectro-optical device free of the foregoing problems.

This object is achieved by a liquid-crystal electro-optical devicecomprising a pair of substrates, a nematic liquid crystal having apositive dielectric anisotropy and interposed between the substrates,and liquid crystal-orienting layers (orientation films) formed on thesubstrates, respectively. These orienting layers are rubbed inantiparallel directions to each other. This device is characterized inthat a molecule of the liquid crystal has a pretilt angle of 4° or less.

An example of such a device is illustrated in FIG. 1. Reference numeral54 designates a pair of polarizing plates, and 51 and 52 designate apair of substrates. The nematic liquid crystal molecule 53 is providedin an electro-optical modulating layer provided between the substrates51 and 52.

The above object is also achieved by a liquid-crystal electro-opticaldevice comprising a pair of substrates, a nematic liquid crystal havinga positive dielectric anisotropy and interposed between the substrates,and liquid crystal-orienting layers formed on the substrates,respectively. These orienting layers are rubbed in antiparalleldirections to each other. This device is characterized in that the polarterm (polar component) of the surface tension at the liquidcrystal-orienting layers is 10 dynes/cm or more.

The above object is also achieved by a liquid-crystal electro-opticaldevice comprising a pair of substrates, a nematic liquid crystal havinga positive dielectric anisotropy and interposed between the substrates,and liquid crystal-orienting layers formed on the substrates,respectively. These orienting layers are rubbed in antiparalleldirections to each other. This device is characterized in that thesurface tension at the liquid crystal-orienting layers is 40 dynes/cm ormore.

We have found that the pretilt angle of the nematic liquid crystal isreduced as the surface tension at the liquid crystal-orienting layers isincreased. The surface tension referred to herein can be expressed inthe form:

surface tension=polar term+dispersion term  (1)

In the present invention, the surface tension is found in the mannerdescribed below.

(1) First, two kinds of liquid i and j having known surface tensions areprepared. For each of these two liquids, the surface tension r_(L), thedispersion term (r_(L))^(d) of the surface tension, and the polar term(r_(L))^(P) of the surface tension are found. Liquids having knownsurface tensions are listed in Table 1.

TABLE 1 r_(L) (r_(L))^(d) (r_(L))^(P) (dynes/cm) (dynes/cm) (dynes/cm)water 72.8 21.8 51.0 glycerin 63.4 37.0 26.4 formamide 58.2 39.5 18.4CH₂I₂ 50.8 48.5 2.3 n-hexadecane 27.6 27.6 0.0

(2) The contact angles are measured with an inspected substrate. Theliquids i and j prepared in item (1) are used at this time. Let X_(i)and X_(j) be their respective contact angles.

(3) Works of adhesion (W_(SL))_(i) and (W_(SL))_(j) are calculated,using equation (2).

(W _(SL))_(i)=(r _(L))_(i) (1+cos X _(i))  (2)

(4) Subsequently, the polar term (r_(S))^(p) and the dispersion term(r_(S))^(d) of the inspected substrate are found from equations (3) and(4).

(W _(SL))_(i)/2=(((r _(L))^(d))_(i))^(½)·((r _(S))^(d))^(½)+(((r_(L))^(p))_(i))^(½)·((r _(S))^(p))^(½)  (3)

(W _(SL))_(j)/2=(((r _(L))^(d))_(j))^(½)·((r _(S))^(d))^(½)+(((r_(L))^(p))_(j))^(½)·((r _(S))^(p))^(½)  (3)

(5) Since the polar term (r_(S))^(p) and the dispersion term (r_(S))^(d)are found in this way, r_(S) can be found from the relationr_(S)=(r_(S))^(d)+(r_(S))^(p).

The liquid crystal used in the present invention can be a cholesteric(chiral-nematic) liquid crystal. However, a nematic liquid crystal ispreferable.

In the prior art twisted-nematic liquid-crystal electro-optical device,the spacing between two substrates is roughly 8 μm. In the presentinvention, the spacing is approximately less than 5 μm, preferably lessthan 3.5 μm.

The “antiparallel directions” referred to herein mean that twosubstrates are rubbed in directions which form an angle of about 180°,as shown in FIG. 1. Therefore, a 90°-twist which would have been inducedin the liquid crystal of the prior art device is not produced. Inconsequence, a display utilizing rotatory polarization as in the priorart techniques cannot be provided. For these reasons, in the presentinvention, a display is provided, by making use of anisotropy of therefractive index of a liquid crystal.

In the present invention, a nematic liquid crystal whose dielectricconstant has positive anisotropy is used. Therefore, it is very easy tocontrol the orientation of the liquid crystal. Furthermore, layers asformed in a smectic liquid crystal are not formed. If the orientation isdisturbed once by an external force, the original orientation isregained quickly after removing the external force. Consequently, it isnot necessary to heat the device until an isotropic or nematic phase isobtained.

The response time of the liquid crystal of the novel device is muchshorter than that of the prior art twisted-nematic liquid crystal. Therise time on application of an electric field is on the order of tens ofmicroseconds, which is comparable to the response times of ferroelectricliquid crystals. Furthermore, the fall time is approximately less than 3milliseconds. Hence, an unparalleled liquid-crystal display can beobtained.

The graph of FIG. 2 shows the relation of the fall time of a liquidcrystal to the pretilt angle of liquid-crystal molecules. As can be seenfrom this graph that where the pretilt angle of the liquid-crystalmolecules is 4° or less, the fall time (the time required for respondingto eliminating an electric field applied to the nematic liquid crystal)is 3 milliseconds or less. Thus, very quick response can be obtained.

The graph of FIG. 3 shows the relation of the fall time of a liquidcrystal to the polar term of the surface tension of orienting layers. Ascan be seen from this graph, where polar term of the surface tension ofthe orienting layers is greater than 10 dynes/cm, the fall time isshorter than 3 milliseconds. In this way, very quick response isderived.

The graph of FIG. 4 shows the relation of the fall time of a liquidcrystal to the surface tension of orienting layers. As can be seen fromthis graph, where the surface tension is greater than 40 dynes/cm, thefall time is shorter than 3 milliseconds, thus giving rise to very quickresponse. The liquid crystal whose characteristics are shown in thegraphs of FIGS. 2-4 is ZLI-4792 manufactured by Merck Corp.

Other objects and features of the invention will appear in the course ofthe description thereof which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating the structure of aliquid-crystal electro-optical device according to the presentinvention;

FIG. 2 is a graph showing the relation of the fall time of a liquidcrystal to pretilt angle;

FIG. 3 is a graph showing the relation of the fall time of a liquidcrystal to the polar term of the surface tension;

FIG. 4 is a graph showing the relation of the fall time of a liquidcrystal to the surface tension;

FIG. 5 is a conceptual diagram illustrating the structure of the priorart twisted-nematic liquid-crystal display; and

FIG. 6 is a schematic optical ray diagram of an optical system usingprojection according to the invention.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

A thin film of ITO (indium tin oxide) 55 was formed on each of a pair ofsubstrates of ground soda-lime glass by sputtering. These thin filmswere patterned photolithographically. Polyimide 56 which is manufacturedunder product name LP by Toray Industries, Inc., Japan, and has a largesurface tension was applied as 1000 Å thick-films to the surfaces onwhich electrodes were to be formed. The surfaces were rubbed. Then, aseal was printed on one surface. A spacer 2 micron thick was dispersedon the other surface.

Thereafter, the surfaces were bonded together in such way that thedirections of rubbing on the substrates had an antiparallel relation toeach other and the spacer was provided between the substrates. Then, aliquid crystal was injected under a vacuum.

After the injection of the liquid crystal, the device was observed witha polarizing microscope. It was found that liquid-crystal molecules wereoriented nearly in the directions of rubbing over the wholeliquid-crystal layer (the whole electro-optical modulating layer).Polarizer plates were stuck to liquid-crystal cell so as to form crossedNicol prisms. At this time, the axis of polarization of the polarizerplate on the incident side was tilted at 45° to the direction ofrubbing. In this manner, the response time was measured. The ITO film isused as a means for applying an electric field to the liquid crystal.The used liquid crystal was the aforementioned ZLI-4792 manufactured byMerck Corp. The rise time was 65 microseconds. The fall time was 2milliseconds. These speeds are about 10 to 1000 times as fast as thespeeds of the prior art twisted-nematic liquid-crystal electro-opticaldevice and comparable to the response times of ferroelectric liquidcrystals.

The pretilt angle of the present liquid-crystal panel was 2°, thesurface tension was 60 dynes/cm, and the polar term was 15 dynes/cm.

Then, a circuit for exciting the liquid crystal was connected. Abacklighting module was assembled. Thus, a transmission-typeliquid-crystal panel was completed.

EXAMPLE 2

In Example 1, fabrication of a transmission-type panel was described. Inthe present example, a reflection type is described. In this case, twopolarizer plates may be used in the same way as in the transmissiontype. However, a display can be provided, using only one polarizerplate. A brighter image can be obtained than possible with a normalreflection type.

In the same way as in Example 1, electrodes of ITO were fabricated on apair of substrates. Then, thin films of polyimide were formed by thesame method as used in Example 1 on the surfaces of the substrates onwhich electrodes were to be formed. The polyimide film on one substratewas rubbed by cotton cloth. SiO₂ particles 1.4 μm in diameter weredispersed as a spacer. This substrate was bonded to the oppositesubstrate on which a seal had been already printed, to form a cell. Thespacing of this cell was measured by well-known interferometry.Subsequently, a nematic liquid crystal was injected into the cell in avacuum. The spacing between the substrates was measured at 5 locations.The obtained values were between 1.3 μm and 1.4 μm.

After sealing the injection port for injecting the liquid crystal, apolarizer plate was stuck to the front surface of the panel. Areflecting plate was stuck to the rear surface. At this time, the axisof polarization of the polarizer plate made an angle of 45° with respectto the axis of rubbing. Then, a circuit for exciting the liquid crystalwas connected. A backlighting module was assembled. Thus, areflection-type liquid-crystal panel was completed. In the presentexample, a display is provided by using only one polarizer plate andthus produces a brighter image than possible with a normalreflection-type liquid-crystal panel.

EXAMPLE 3

In the present example, the present invention is applied to a colorprojector, which is described below by referring to FIG. 6. This colorprojector uses no color filters and so this projector is brighter andhas a higher transmittance than the prior art projector.

Referring to FIG. 6, a He-Ne laser having a peak near wavelength 633 nmwas used as a red light source 61. An Ar laser having a peak nearwavelength 515 nm was employed as a green light source 62. Also, an Arlaser having a peak near wavelength 477 nm was used as a blue lightsource 63. The contours and the optical densities of these laser beamswere adjusted according to required optical conditions by optics 64, andthese beams were made to irradiate liquid-crystal panels 65, 66, and 67fabricated in Example 1.

The light passed through each liquid-crystal panel is turned on and offor its amount of light is restricted by the shutter function of theliquid-crystal panel to form a gray scale. The three kinds of light arecombined by an optical system 68 and projected onto a display screen 69to an enlarged scale.

In the present example, each light source emits a single wavelength and,therefore, the optical conditions for light passing through thecorresponding liquid-crystal panel can be matched to this panel. Hence,the amount of scattering light after passage through the panel is small.The enlarged and projected image is not blurred. As a result, a sharpdisplay can be provided.

Examples of lasers which can be used in the present example includeaforementioned gas lasers, lasers using vapors of a metal such ascadmium or zinc, solid-state lasers, and other lasers having peakwavelengths in the visible region. Furthermore, even wavelengths otherthan normal operating wavelengths can be used if visible light can beobtained by passing them through a special optical system. For instance,the second harmonic of a YAG laser can be used.

Normally, three laser beams having wavelengths close to the wavelengthsof red, blue, and green light are employed. Four or more laser beamshaving different wavelengths may be used, and colors may be synthesizedto display an image in color.

As described thus far, the novel liquid-crystal electro-optical displaycan provide a display in a mode which has never been encountered in theprior art liquid-crystal electro-optical display. In the presentinvention, the orientation of the liquid crystal can be very easilycontrolled. Furthermore, the response speeds, especially the fallingcharacteristics, of the novel liquid-crystal electro-optical device arequite excellent. Moreover, the invention facilitates fabricating a largedisplay screen.

What is claimed is:
 1. An electro-optical device comprising: a pair oforientation films provided over a pair of substrates respectively andhaving antiparallel orientation directions to each other; and anelectro-optical modulating layer provided between said pair ofsubstrates and comprising a nematic liquid crystal having a positivedielectric anisotropy, said liquid crystal being composed of moleculesaligned substantially in one direction throughout a thickness of saidelectro-optical modulating layer, wherein said molecules of said nematicliquid crystal have a pre-tilt angle of 4° or less.
 2. A deviceaccording to claim 1 further comprising: a first electrode provided overone of said substrates; and a second electrode provided over the otherof said substrates.
 3. A device according to claim 1 wherein saidorientation directions are rubbing directions.
 4. A device according toclaim 1 wherein said orientation films comprise polyimide.
 5. A grayscale electro-optical device comprising: a pair of orientation filmsprovided over a pair of substrates respectively and having antiparallelorientation directions to each other; and an electro-optical modulatinglayer provided between said pair of substrates and comprising a nematicliquid crystal having a positive dielectric anisotropy, said liquidcrystal being composed of molecules aligned substantially in onedirection throughout a thickness of said electro-optical modulatinglayer, wherein said molecules of said nematic liquid crystal have apre-tilt angle of 4° or less.
 6. A device according to claim 5 furthercomprising: a first electrode provided over one of said substrates; anda second electrode provided over the other of said substrates.
 7. Adevice according to claim 5 wherein said orientation directions arerubbing directions.
 8. A device according to claim 5 wherein saidorientation films comprise polyimide.
 9. A device according to claim 5wherein an interval between said substrates is 5 μm or less.
 10. Anelectro-optical device comprising: a pair of orientation films providedover a pair of substrates respectively and having antiparallelorientation directions to each other; and an electro-optical modulatinglayer provided between said pair of substrates and comprising a nematicliquid crystal having a positive dielectric anisotropy, said liquidcrystal being composed of molecules aligned substantially in onedirection throughout a thickness of said electro-optical modulatinglayer, wherein said molecules of said nematic liquid crystal have apre-tilt angle of 4° or less, and wherein said orientation films have apolar component of a surface tension in the range of 10 dyne/cm or more.11. A device according to claim 10 further comprising: a first electrodeprovided over one of said substrates; and a second electrode providedover the other of said substrates.
 12. A device according to claim 10wherein said orientation directions are rubbing directions.
 13. A deviceaccording to claim 10 wherein said orientation films comprise polyimide.14. An electro-optical device comprising: a pair of orientation filmsprovided over a pair of substrates respectively and having antiparallelorientation directions to each other; and an electro-optical modulatinglayer provided between said pair of substrates and comprising a nematicliquid crystal having a positive dielectric anisotropy, said liquidcrystal being composed of molecules aligned substantially in onedirection throughout a thickness of said electro-optical modulatinglayer, wherein said molecules of said nematic liquid crystal have apre-tilt angle of 4° or less, and wherein said orientation films have asurface tension in the range of 40 dyne/cm or more.
 15. A deviceaccording to claim 14 further comprising: a first electrode providedover one of said substrates; and a second electrode provided over theother of said substrates.
 16. A device according to claim 14 whereinsaid orientation directions are rubbing directions.
 17. A deviceaccording to claim 14 wherein said orientation films comprise polyimide.18. A reflection-type electro-optical device comprising: a pair oforientation films provided over a pair of substrates respectively andhaving antiparallel orientation directions to each other; anelectro-optical modulating layer provided between said pair ofsubstrates and comprising a nematic liquid crystal having a positivedielectric anisotropy, said liquid crystal being composed of moleculesaligned substantially in one direction throughout a thickness of saidelectro-optical modulating layer; and a reflecting plate provided on arear side of one of said substrates, wherein said molecules of saidnematic liquid crystal have a pre-tilt angle of 4° or less.
 19. A deviceaccording to claim 18 further comprising: a first electrode providedover one of said substrates; and a second electrode provided over theother of said substrates.
 20. A device according to claim 18 whereinsaid orientation directions are rubbing directions.
 21. A deviceaccording to claim 18 wherein said orientation films comprise polyimide.22. A gray scale electro-optical device comprising: a pair oforientation films provided over a pair of substrates respectively andhaving antiparallel orientation directions to each other; and anelectro-optical modulating layer provided between said pair ofsubstrates and comprising a nematic liquid crystal having a positivedielectric anisotropy, wherein molecules of said nematic liquid crystalhave a pre-tilt angle of 4° or less.
 23. A device according to claim 22further comprising: a first electrode provided over one of saidsubstrates; and a second electrode provided over the other of saidsubstrates.
 24. A device according to claim 22 wherein said orientationdirections are rubbing directions.
 25. A device according to claim 22wherein said orientation films comprise polyimide.
 26. A deviceaccording to claim 22 wherein an interval between said substrates is 5μm or less.